Changes in root morphological traits in soybean co-inoculated with Bradyrhizobium spp. and Azospirillum brasilense or treated with A. brasilense exudates

We assessed the effects of co-inoculation with Azospirillum brasilense or the application of its exudates via seeds or leaf spray on root morphological traits and nodulation, as well as on shoot development and grain yield of soybean inoculated with Bradyrhizobium. Two experiments were performed in sterile substrate under greenhouse, and two were performed at field conditions in sandy soils in a crop season with episodes of drought. The treatments in the greenhouse experiments comprised the non-inoculated control, sole inoculation with Bradyrhizobium, co-inoculation of Bradyrhizobium and A. brasilense, and inoculation of Bradyrhizobium with the application of Azospirillum exudates via seeds or leaf spray. Field treatments included non-inoculated controls without and with N-fertilizer and inoculation of Bradyrhizobium and co-inoculation with A. brasilense. Plants were assessed for root weight, total and specific lengths, volume, diameter, tissue density, branches number, root-hair length and incidence, nodule number and dry weight, shoot dry weight and N content, and grain yield. Co-inoculation of Bradyrhizobium and A. brasilense and seed application of A. brasilense exudates increased the number of root branches and nodules compared with the sole inoculation of Bradyrhizobium. However, leaf spray application of exudates was less effective. Co-inoculation also increased specific root length, root length density in soil, root-hair incidence and length, and total N content in shoot, altogether resulting in increases in grain yield. Co-inoculation of soybean with Bradyrhizobium and A. brasilense improved several root morphological traits, increasing the plant capacity to overcome moderate drought stress episodes in sandy soils, allowing to reach higher yields.